Conventionally, as an image forming (printing) operation in an electrophotographic image forming apparatus such as a laser printer, an electrostatic latent image is formed on a photosensitive drum based on image data sent from a host apparatus such as a personal computer (hereinafter, referred to as simply “PC”), toner is affixed to this electrostatic latent image, and image data is made manifest on the photosensitive drum. Afterwards, recording paper transported on a paper transport path is passed between the photosensitive drum and a transfer roller, and the toner image on the photosensitive drum is transferred to the front side of the recording paper. Then, this recording paper passes a fixing roller, and the toner image is fixed by the fixing roller onto the recording paper with heat and pressure.
In recent years, the development of image forming apparatuses of this type provided with a duplex printing function that performs printing on both the front side and the back side of the recording paper has been advancing. The two methods described below are known as methods for this duplex printing.
In the first method, when duplex printing of multiple sheets is requested, among the print data of the front and back side requested to be printed, first, only front side printing (simplex printing) is successively performed across multiple sheets of recording paper with the front-side print data, and these multiple sheets of recording paper for which front side printing is complete are stored in a middle tray. Then, after all front side printing with the front side print data is complete, back side printing is performed for each page of recording paper with back side image data while taking out recording paper stored in the middle tray page by page, and by doing so multiple sheets of recording paper which have been printed on both sides are successively discharged to a discharge tray.
On the other hand, the second method includes, when duplex printing is requested, printing with front and back image data one page at a time. In order to perform duplex printing with this method, it is necessary for a primary transport path and a secondary transport path to be provided as a transport path that transports the recording paper, and to have a recording paper switchback function. That is, after transporting the paper to the primary transport path and performing front side printing, that recording paper is transported to the secondary transport path via the switchback function. Thus the front and back of the recording paper are reversed, the recording paper is again transported to the primary transport path, and printing is performed on the back side of the recording paper.
With the first of the two methods described above, a middle tray is necessary of a size that can store multiple sheets of recording paper, in order to handle the case that multiple sheets of duplex printing are requested. That is, because a large middle tray is necessary, it becomes difficult to achieve a compact design for the image forming apparatus.
Also, with this first method, when multiple sheets of duplex printing are requested, because back side printing is begun after front side printing of all of these multiple sheets is completed, a long time is required until the user obtains recording paper that has been printed on both sides (until the print status of the first page of printed material can be confirmed).
Because there are the problems described above in the first method, in recent years, various image forming apparatuses that perform duplex printing with the second method described above have been developed.
When performing duplex printing with an image forming apparatus using this second method, as stated above, after a toner image is transferred to one side of one sheet of the recording paper, a fixing process is performed that fixes the toner image to that recording paper, and afterwards, back side printing subsequently begins. Because heat fixing is generally performed in the fixing process, the moisture content of the surface of the recording paper used in this fixing process is reduced by fixing heat. As a result, the surface resistance value of the recording paper when performing back side printing is increased in comparison to the surface resistance value of the recording paper when performing front side printing.
When using, for example, A4 size recording paper (below, simply “paper”), the surface resistance value of the paper changes as shown in FIG. 5. That is, during the transfer process (1) in front side printing of the first sheet of paper, the surface resistance value of the paper is 1×108 Ω·cm. In the fixing process (1) performed immediately afterward, the moisture content of the paper surface is lost due to the fixing heat, and the surface resistance value of the paper increases to about 0.5×1010 Ω·cm. Afterwards, when switching back the paper (switchback transfer process) in order to perform back side printing of the first sheet of paper, the moisture content inside the paper swells to the paper surface, and thus the surface resistance value of the paper decreases slightly to about 1×109 Ω·cm. Therefore, in the transfer process (2) when printing the back side, transfer is performed to paper having a surface resistance value of 1×109 Ω·cm, but due to going through the subsequent fixing process (2), the surface resistance value of the paper rises again. Also when performing duplex printing of the second and subsequent sheets of paper, similarly, by going through the processes described above, the surface resistance value of the paper changes (see FIG. 5, transfer process (3)—discharge of second sheet of paper complete).
Ordinarily, a constant current control is performed in the transfer process that controls the transfer operation with a constant electric current, but as described above, when performing transfer for duplex printing, when executing a transfer operation with a constant current control to identical paper of a different surface resistance value, the transfer voltage applied to the transfer roller when performing the transfer process is vastly different for front side printing and back side printing. The surface resistance value of the paper (ordinarily, about 1×106 to 1×1010 Ω·cm) changes about 1×101 to 1×102 Ω·cm depending on the size of the paper, the moisture content of the paper, the surrounding environment, and the like, and due to the large change in the surface resistance value of the paper that accompanies the fixing process as described above, the transfer voltage when printing the back side may be an applied voltage nearly two times the transfer voltage when printing the front side. This sort of difference in transfer voltage exerts a large influence on the transfer properties for front side printing and back side printing, and invites defects wherein the print quality of the front side and the back side is not the same.
On the other hand, when duplex printing is performed for multiple sheets, when printing the front side of the second and subsequent sheets, print defects are generated due to “photographic fog”, described below. Following is an explanation of the circumstances in which this “photographic fog” is generated.
As shown in FIG. 6(a), the paper is sandwiched between a transfer roller a and a photosensitive body b, and when transferring the toner image of the photosensitive body b to the paper, the transfer current that flows in the direction of the photosensitive body (electric current of the transfer roller) in the transfer process for the front side of the first sheet of paper is about constant (see the solid line in FIG. 6(b)).
Thus, as shown by the solid line in FIG. 6(c), the surface electric potential of the photosensitive body b after the transfer process for the front side of the first sheet of paper has been performed (immediately before performing the transfer process for the back side of the first sheet of paper) is roughly constant across the entire photosensitive body b.
However, when performing the transfer process for the back side of the first sheet of paper, because the paper has been through the fixing process when printing the front side, as described above, the resistance value of the paper has increased, and because of that increase in the resistance value the transfer current cannot easily flow. As a method for eliminating such a problem, there is the constant current control system described above that always lets a constant current flow. This method attempts to maintain a constant current by increasing the voltage by the extent that it is difficult for the current to flow.
On the other hand, in a portion h outside the region that paper is arranged (a portion separate from the paper passage region), paper does not lie between the photosensitive body and the transfer roller, current flows easily because resistance is the same as under the condition when performing the transfer process for the front side of the first sheet, and in comparison to when performing the transfer process for the front side of the first sheet, a large amount of current flows on the photosensitive body b outside of the region where paper is arranged. The voltage at this time is the same as for the region where paper is arranged (paper passage region), and is a higher voltage than when performing the transfer process to the front side of the first sheet.
Due to this phenomenon, on the photosensitive body b outside the region where the paper is arranged, the photosensitive body charging and reverse polarity transfer current greatly flows in with a high voltage condition, and as a result the charging potential of the photosensitive body b decreases (a canceling phenomenon due to reverse potential occurs).
Thus, as shown by the broken line in FIG. 6 (c), in the photosensitive body b after performing the transfer process for the back side of the first sheet of paper (immediately before performing transfer for the front side of the second sheet of paper), the surface potential of both edges that are outside the region where the paper is arranged decreases. Thus naturally, even in a non-image portion that prevents toner affixing due to the potential difference between the toner and the photosensitive body b, a phenomenon occurs wherein toner is unintentionally affixed onto the photosensitive body b due to the small difference in electric potential (this phenomenon is called photographic fog).
In this way, a state in which photographic fog is generated on the photosensitive body b (a state in which toner is affixed because the surface potential is decreased) continues until the recharging process is performed, that is, until the photosensitive body completes at least one full turn. In this state, when the second page of paper is transported, a fogged image is transferred to the leading edge position corresponding within one full turn of the photosensitive body, and this state is generated when shifted in the axial direction of the photosensitive body relative to the paper passage position due to paper transport variation (positional displacement in the width direction of the paper) when performing back side transfer for the first sheet of paper.
Accordingly, in order to eliminate transfer defects and printing defects in duplex printing as described above, image forming apparatuses have been proposed that control voltage such that the transfer voltage is constant, and do not perform the transfer process with a constant current control (for example, see JP 2002-49184A). Image forming apparatuses have also been proposed that perform the transfer process by constant current control, reduce residual electric potential on the photosensitive body by de-electrifying the photosensitive body, and make the transfer voltage constant (for example, see JP 2002-23576A).
However, when making the transfer voltage constant by the methods disclosed in JP 2002-49184A and JP 2002-23576A, there is much damage to the photosensitive body, inviting a deterioration in the life properties of the photosensitive body. That is, with the image forming apparatus disclosed in JP 2002-23576A, because de-electrification of the photosensitive body is performed with a de-electrifying voltage of opposite polarity to the charging properties of the photosensitive body, it is possible that this will lead to a deterioration in the life properties of the photosensitive body. Also, with the image forming apparatus disclosed in JP 2002-49184A, change occurs in the resistance value of the printing paper due to changing the environment of the apparatus for constant voltage control, and a change in the optimum voltage occurs. The voltage to the photosensitive body changes, damage is conferred on the photosensitive body, and in this case as well leads to a deterioration in the life properties of the photosensitive body.
The present invention was made in consideration of the circumstances described above, and it is an object thereof to provide an image forming apparatus provided with a duplex printing function, wherein a deterioration in life properties of the photosensitive body is not invited, and wherein print defects due to “photographic fog” can be decreased.